Method for preparing volatile fatty acids by fermenting kitchen waste and application thereof

Abstract

The present application relates to the technical field of environmental protection, and particularly relates to a method for preparing volatile fatty acids by fermenting kitchen waste and application thereof. The method comprises: performing alkalization pretreatment on kitchen waste materials, increasing the pH value of the kitchen waste materials to an alkalization pretreatment pH value, and performing a first-stage alkaline hydrolysis reaction under the condition to obtain pretreated kitchen waste substrate; adjusting the pH value of the pretreated kitchen waste substrate to a post-adjustment fermentation pH value, then mixing the pretreated inoculum, and performing a second-stage anaerobic fermentation reaction under a controlled environment condition; wherein the alkalization pretreatment pH value is higher than the post-adjustment fermentation pH value. The method provided by the present application can independently control the reaction conditions of each stage, takes into account the substrate hydrolysis and volatile fatty acid synthesis efficiency, is flexible in process operation, does not require complex additional links, and also provides a process basis for the directional synthesis of specific volatile fatty acids, and is strong in adaptability and practicality.

Description

Technical Field

[0001] This invention relates to the field of environmental protection technology, and in particular to a method for preparing volatile fatty acids by fermentation of kitchen waste and its application. Background Technology

[0002] Food waste is a typical type of easily perishable organic solid waste, and its resource utilization is an important research direction in the field of solid waste treatment. Acidic anaerobic fermentation technology can convert degradable organic matter in food waste into volatile fatty acids (VFAs) under conditions that inhibit the activity of methanogens. These substances contain various components such as acetic acid, propionic acid, and butyric acid, and are important platform chemicals for the synthesis of biofuels and bio-based chemicals. They can be further derived into various industrial products, providing an effective pathway for the high-value resource recovery of food waste. Therefore, the technology of preparing volatile fatty acids from food waste through acidic anaerobic fermentation has received widespread attention and research.

[0003] In existing technologies for the fermentation of food waste to produce volatile fatty acids, researchers primarily optimize fermentation effects and increase the overall yield of volatile fatty acids through substrate regulation, pH / alkalinity control, inhibition of methanogens, and microbial screening and acclimatization. pH regulation is one of the core control methods. Current technologies mainly focus on pH control in the fermentation system or pretreatment stage within the weakly acidic to moderately alkaline range, with no research yet elevating the pH to the strongly alkaline range of 13.3 or higher. Conventional process understanding often focuses on regulating mild acid-base conditions, aiming to improve substrate hydrolysis efficiency and protect the activity of functional microorganisms through appropriate pH adjustments, laying the foundation for subsequent fermentation synthesis of volatile fatty acids. However, current technologies still face the challenge of producing a mixture of various volatile fatty acids as fermentation products, failing to achieve the targeted synthesis of a single target fatty acid, and also bringing many inconveniences to the subsequent separation and purification of fatty acids. Summary of the Invention

[0004] To address the shortcomings of the prior art, this invention provides a method for preparing volatile fatty acids through fermentation of kitchen waste and its application.

[0005] The first aspect of this invention provides a method for preparing volatile fatty acids through fermentation of kitchen waste, comprising the following steps: Alkalinization pretreatment is carried out on the food waste material to raise its pH value to the alkaline pretreatment pH, and the first stage of alkaline hydrolysis reaction is carried out under this condition to obtain the pretreated food waste substrate; The pH value of the pretreated kitchen waste substrate was adjusted to the pH value after the initial fermentation, and then mixed with the pretreated inoculum to carry out the second stage of anaerobic fermentation under controlled environmental conditions. The pH of the alkaline pretreatment is higher than the pH of the post-fermentation after the adjustment.

[0006] In one embodiment of the present invention, the pH of the alkaline pretreatment is above 13.3; Preferably, the pH of the alkaline pretreatment is 13.3 to 13.5.

[0007] In one embodiment of the present invention, the post-fermentation pH is 8.0 to 10.0.

[0008] In one embodiment of the present invention, the duration of the first-stage alkaline hydrolysis reaction is 0.5 hours to 1 hour.

[0009] In one embodiment of the present invention, the inoculum is anaerobic sludge that has undergone preheating treatment; Preferably, the temperature range of the preheating treatment is 90°C to 105°C, and the treatment time is 1 hour to 2 hours.

[0010] In one embodiment of the present invention, the total solids content of the inoculum after preheating treatment is 10% to 30%.

[0011] In one embodiment of the present invention, the ratio of the pretreated food waste substrate to the inoculum is 1g of inoculum for every 10 to 12ml of substrate.

[0012] In one embodiment of the present invention, the second-stage anaerobic fermentation reaction is carried out under an inert gas atmosphere.

[0013] In one embodiment of the present invention, the second-stage anaerobic fermentation reaction is carried out in a closed reactor; Preferably, the reaction temperature of the second-stage anaerobic fermentation reaction is 35°C to 36°C; Preferably, the total duration of the second-stage anaerobic fermentation reaction is 4 to 10 days.

[0014] A second aspect of the present invention provides the application of the method as described in the first aspect in the directed fermentation of food waste to prepare propionic acid.

[0015] Based on the above, compared with existing technologies, this invention utilizes a two-stage process of alkali pretreatment hydrolysis followed by pH adjustment anaerobic fermentation, with the alkaline pretreatment pH being higher than the fermentation pH, to achieve efficient regulation of volatile fatty acid production from food waste. Strong alkaline pretreatment efficiently hydrolyzes large-molecule organic matter in food waste, improving substrate biodegradability; pH ​​adjustment creates a suitable microenvironment for fermentation, protecting the activity of functional microorganisms.

[0016] Other features and beneficial effects of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other beneficial effects of the invention can be realized and obtained by means of the structures particularly pointed out in the description, claims and drawings. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Unless otherwise specified, the positional relationships shown in the drawings in the following description are based on the direction in which the components are drawn in the figure.

[0018] Figure 1 The diagram shown is a process flow chart of the method for preparing volatile fatty acids by fermentation of kitchen waste provided by the present invention. Figure 2 The figure shows the changes in total acid yield of fermentation under different pretreatment pH and pullback pH in the embodiments and comparative examples of the present invention. Figure 3 The figure shows the changes in propionic acid production during fermentation at different pretreatment pH and adjustment pH in the embodiments and comparative examples of the present invention. Figure 4 The figures show the changes in propionic acid selectivity during fermentation at different pretreatment pH and pullback pH in the embodiments and comparative examples of the present invention. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. The technical features designed in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0020] In the description of this invention, it should be noted that all terms used in this invention (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains, and should not be construed as limiting the invention; it should be further understood that the terms used in this invention should be understood to have the same meaning as those in the context of this specification and in the relevant field, and should not be understood in an idealized or overly formal sense, except as expressly defined in this invention.

[0021] Those skilled in the art will understand that the order in which the steps are written in the various implementations or embodiments does not imply a strict execution order and does not constitute any limitation on the implementation process. The detailed execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps in this application can be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps 1 and 2, it means that the method may include steps 1 and 2 performed sequentially, or it may include steps 2 and 1 performed sequentially. For example, if the method may also include step 3, it means that step 3 can be added to the method in any order. For example, the method may include steps 1, 2, and 3, or it may include steps 1, 3, and 2, or it may include steps 3, 1, and 2, etc.

[0022] An embodiment of the first aspect of the present invention provides a method for preparing volatile fatty acids by fermentation of kitchen waste, comprising the following steps: Step 1: Perform alkalization pretreatment on the kitchen waste material to raise its pH value to the alkaline pretreatment pH, and carry out the first stage of alkaline hydrolysis reaction under this condition to obtain the pretreated kitchen waste substrate; In practice, a certain amount of kitchen waste (taken from the oil-removed storage tank of Xiamen Ruikeji Renewable Energy Co., Ltd., with a total solids content (TS) of approximately 8%-10%) can be placed in a pretreatment container. Under continuous mechanical stirring (e.g., using a paddle mixer at a speed of 100-200 rpm), a high-concentration alkaline solution is slowly added to raise and stabilize the pH of the mixture at the alkaline pretreatment pH.

[0023] It is worth noting that the alkaline solution can be selected from one or a mixture of several of sodium hydroxide, potassium hydroxide, and lime milk (calcium hydroxide suspension), with sodium hydroxide solution being preferred due to its lower cost and higher solubility. Under this pH condition, stirring is maintained to carry out the first stage of alkaline hydrolysis reaction. The reaction time is sufficient for complex organic matter (such as proteins, fats, and carbohydrates) to undergo saponification, degradation, and other reactions, resulting in a homogeneous and fully hydrolyzed pretreated food waste substrate. The stirring method is not limited to mechanical stirring; ultrasonic-assisted stirring, pneumatic mixing, and other effective methods can also be used to enhance mass transfer and reaction.

[0024] Step 2: Adjust the pH value of the pretreated kitchen waste substrate to the pH value after the initial fermentation, and then mix it with the pretreated inoculum to carry out the second stage of anaerobic fermentation under controlled environmental conditions. The pH of the alkaline pretreatment is higher than the pH of the post-fermentation after the adjustment.

[0025] In practice, a dilute acid solution is slowly added to the pretreated kitchen waste substrate obtained in step 1 to adjust the pH of the system to the pH level after the initial fermentation. Then, the pretreated inoculum is added to the pH-adjusted substrate. The mixed materials are transferred to a closed reactor (such as a serum bottle or anaerobic fermenter), and an inert gas (such as high-purity nitrogen or argon) is introduced to replace the air at the top of the reactor for about 5-10 minutes to establish an inert gas atmosphere and ensure an anaerobic environment.

[0026] The reactor was then sealed and placed in a constant temperature incubator or water bath to carry out the second stage of anaerobic fermentation under controlled environmental conditions. During fermentation, samples were intermittently taken to monitor indicators such as pH and volatile fatty acid composition.

[0027] The alkaline pretreatment pH is higher than the post-fermentation pH. This first high and then low pH change sequence first uses strong alkaline conditions to achieve complete hydrolysis of kitchen waste and possibly inhibit some miscellaneous bacteria, and then adjusts to a pH window suitable for the metabolism of acid-producing bacteria (especially propionic acid-producing bacteria), thereby guiding the fermentation product profile.

[0028] It is worth noting that the dilute acid can be selected from inorganic or organic acids such as hydrochloric acid, sulfuric acid, phosphoric acid, and acetic acid, with hydrochloric acid or phosphoric acid being preferred as they have minimal impact on subsequent microbial metabolism. The inoculum is anaerobic sludge that has undergone heat pretreatment to inactivate non-target bacterial groups such as methanogens.

[0029] This invention employs a two-step pH control strategy—pretreatment with strong alkali (pH ≥ 13.3) followed by adjustment to a suitable fermentation pH (8.0-10.0)—breaking away from the traditional technical bias in the field of avoiding strong alkaline conditions to maintain microbial activity. This method achieves deep hydrolysis and targeted regulation of the microbial community in a single reaction system without relying on exogenous pure bacterial inoculation or complex multi-stage processes. It significantly improves the selectivity (proportion) and total yield of propionic acid in volatile fatty acid fermentation products, while maintaining a simple process and controllable operating costs. This provides a novel technical pathway for the efficient conversion of food waste into a high-value-added single chemical, greatly enhancing the economic feasibility of its resource utilization.

[0030] In this invention, the inoculum specifically refers to a selectively pretreated source of active microorganisms introduced to initiate and dominate the anaerobic acid-producing fermentation of food waste. A typical and preferred embodiment is anaerobic sludge that has undergone heat pretreatment (e.g., treatment at 90-105°C for 1-2 hours). This treatment aims to selectively inactivate or strongly inhibit non-target microorganisms such as methanogenic archaea, while retaining or screening functional microorganisms with acid-producing metabolic capabilities (especially heat resistance), thereby creating a starting microbial community conducive to the accumulation of volatile fatty acids (especially propionic acid) after inoculation. The total solids (TS) content of the inoculum is typically adjusted to between 10% and 30%, and it is mixed with the pretreated food waste substrate at a specific volume-to-mass ratio (e.g., 10-12 ml substrate: 1 g inoculum). Those skilled in the art should understand that the core functional characteristic of the inoculum lies in the methane-inhibiting and acid-producing microbial community structure it possesses after pretreatment. Its specific source, pretreatment method (not limited to heat treatment), and physical morphology, as well as various changes and substitutions made without deviating from this core function, all fall within the protection scope of this invention.

[0031] It should also be noted that the controlled environmental conditions described in this invention are a general description of the reaction system in which the second-stage anaerobic fermentation reaction takes place, and which has undergone artificial intervention and regulation. Specifically, it refers to the control of key parameters of the reaction system to promote the directed fermentation of food waste to produce acid, particularly to improve propionic acid selectivity. These controls include, but are not limited to: establishing and maintaining a strictly anaerobic atmosphere using inert gases; using a closed reactor to isolate air and contaminants; and maintaining the reaction temperature constant within a range suitable for the metabolism of mesophilic acid-producing bacteria (e.g., 35°C to 36°C). Furthermore, it may also involve monitoring the pH stability of the reaction system and appropriate mixing to ensure mass transfer. Those skilled in the art can make equivalent substitutions or adaptive adjustments to the specific implementation of the above environmental conditions according to the actual production scale and equipment, without departing from the core concept of this invention.

[0032] Furthermore, the method described in this invention has broad applicability to the sources and composition of food waste. It broadly refers to easily perishable organic waste generated during food processing, catering services, and household consumption in places such as households, catering businesses, and company canteens. Its typical components include, but are not limited to: starchy food residues (such as rice and pasta), protein residues (such as meat, fish, and soy products), fat residues (such as animal and vegetable oils), vegetable and fruit residues (such as vegetable leaves and fruit peels), and small amounts of discarded tableware, paper towels, and other impurities. Regardless of the specific compositional fluctuations of food waste, as long as it contains biodegradable organic matter, it can be treated using the "alkali pretreatment-pH adjustment" process described in this invention, achieving highly selective production of volatile fatty acids, especially propionic acid. Simple crushing, screening (removing large impurities), and homogenization steps before pretreatment help improve the uniformity of the treatment, but are not essential limitations of this invention.

[0033] In a preferred embodiment of the invention, the alkaline pretreatment pH is above 13.3; preferably, the alkaline pretreatment pH is 13.3 to 13.5. Specifically, a 4 M sodium hydroxide solution can be used to adjust the pH of the food waste material to 13.3-13.5, and a pH meter can be used for real-time monitoring to ensure stability. This strongly alkaline environment effectively promotes oil saponification, protein denaturation and hydrolysis, and the destruction of the lignocellulose structure, significantly improving the biodegradability of the substrate and creating conditions for the subsequent selective inhibition of certain microbial populations. After pretreatment at this preferred pH range, the selectivity of propionic acid in the subsequent fermentation products is significantly higher than that under pretreatment at lower pH (e.g., 10 or 12).

[0034] Furthermore, the pH of the alkaline pretreatment can be 13.3, 13.4, 13.5, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. The alkali used can also be replaced with an equivalent amount of potassium hydroxide solution, or a mixed solution of sodium hydroxide and potassium hydroxide.

[0035] In a preferred embodiment of the present invention, the pH of the post-fermentation adjustment is 8.0 to 10.0. Specifically, a 1 M hydrochloric acid solution is used to adjust the pH of the pretreated kitchen waste substrate to 8.0-10.0. This pH range is within the range where many acid-producing bacteria, especially certain propionic acid-producing bacteria, have high metabolic activity, while effectively inhibiting the activity of methanogenic bacteria.

[0036] Furthermore, the post-fermentation pH can be 8.0, 8.5, 9.0, 9.5, 10.0, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable. The acid used for the post-fermentation can also be replaced with sulfuric acid, phosphoric acid, or organic acids (such as acetic acid) depending on cost and subsequent process requirements.

[0037] In a preferred embodiment of the present invention, the duration of the first-stage alkaline hydrolysis reaction is 0.5 hours to 1 hour. The duration can be 0.5 hours, 0.75 hours, 1 hour, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. This time is sufficient to ensure adequate contact between the alkali solution and the material to complete the main hydrolysis reaction, while avoiding excessively long durations that could lead to unnecessary energy consumption increases or the potential generation of inhibitory byproducts.

[0038] In a preferred embodiment of the present invention, the inoculum is preheated anaerobic sludge; preferably, the preheating temperature range is 90°C to 105°C, and the treatment time is 1 to 2 hours. Specifically, residual sludge from the anaerobic digestion stage of a municipal wastewater treatment plant is taken and heated in an oven or water bath at 105°C for 2 hours. This preheating treatment aims to kill or severely inhibit methanogenic archaea and other non-target microorganisms in the sludge, while retaining or screening for heat-resistant acid-producing bacteria (including Bacillus), thereby enhancing the acid-producing fermentation process and inhibiting methane production after inoculation.

[0039] Furthermore, the preheating temperature can be 90℃, 95℃, 100℃, 105℃, etc., and the treatment time can be 1 hour, 1.5 hours, 2 hours, etc., but is not limited to the listed values; other unlisted values ​​within their respective ranges are also applicable. The heat source can be an oven, a water bath, steam heating, etc.

[0040] In a preferred embodiment of the present invention, the total solids content of the preheated inoculum is 10% to 30%. Specifically, the preheated anaerobic sludge is appropriately concentrated through natural sedimentation or low-speed centrifugation to adjust its total solids (TS) content to approximately 10%-30%. A suitable TS content ensures sufficient microbial biomass in the inoculum while avoiding mass transfer problems caused by excessively high solids content or insufficient inoculum due to excessively low solids content. The principle is to optimize the microbial inoculation density. The technical effect is to ensure rapid fermentation start-up and stable progress. Furthermore, the total solids content can be 10%, 15%, 20%, 25%, 30%, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0041] In a preferred embodiment of the present invention, the ratio of the pretreated food waste substrate to the inoculum is 1 g of inoculum for every 10 to 12 ml of substrate. Specifically, the food waste substrate after pH adjustment and the preheated concentrated anaerobic sludge (TS approximately 10%-30%) are mixed in the reactor at a volume-to-mass ratio of 12:1 (ml:g). This ratio provides optimized substrate and microbial load, which is beneficial for efficient fermentation.

[0042] Furthermore, the dosage ratio can be 10:1, 11:1, 12:1 (ml:g), etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0043] In a preferred embodiment of the present invention, the second-stage anaerobic fermentation reaction is carried out under an inert gas atmosphere. The inert gas may be argon, helium, or a mixture thereof; in addition to the purging method, a vacuum replacement method (vacuuming followed by filling with inert gas) may also be used to establish the anaerobic environment.

[0044] In a preferred embodiment of the invention, the second-stage anaerobic fermentation reaction is carried out in a closed reactor; preferably, the reaction temperature of the second-stage anaerobic fermentation reaction is 35°C to 36°C; preferably, the total duration of the second-stage anaerobic fermentation reaction is 4 to 10 days. The closed reactor prevents gas leakage and external contamination. A certain fermentation duration ensures the full conversion of the substrate and the accumulation of the target product. The principle is to provide stable and suitable biological reaction conditions, thereby achieving the efficient and stable production of volatile fatty acids, especially propionic acid.

[0045] Furthermore, the reaction temperature can be 35℃, 35.5℃, 36℃, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. The total reaction time can be 4 days, 6 days, 8 days, 10 days, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. The optimal selective propionic acid production time point occurs around day 4. The reactor can also be a small-scale anaerobic fermenter, anaerobic tube, or other closed containers.

[0046] A second aspect of the present invention provides the application of the method as described in the first aspect in the directed fermentation of food waste to prepare propionic acid.

[0047] Among the various VFAs prepared by fermentation of food waste, propionic acid has attracted much attention due to its specific application needs in food preservation, feed additives, chemical synthesis and other fields. In order to increase the proportion of propionic acid in mixed acids (i.e. propionic acid selectivity), existing technologies and patents have explored many aspects, mainly including: (1) substrate regulation, such as supplementing carbon sources that are easy to generate propionic acid (such as lactic acid); (2) process parameter optimization, such as fermentation within a specific pH range (commonly pH 5-9) to promote the dominant growth of propionic acid-producing bacteria; (3) microbial engineering, such as inoculating specific pure bacteria such as Propionibacterium acnes for fermentation; (4) multi-stage coupled processes, such as first carrying out the lactic acid production stage and then carrying out the lactic acid to propionic acid conversion stage.

[0048] For example, some existing technologies can increase the proportion of propionic acid in total VFA to approximately 45% by controlling the fermentation pH to near neutral and adjusting the substrate. Other patented technologies employ a "pre-acidification + pure culture secondary fermentation" model, enriching propionic acid by inoculating the fermentation broth with Propionibacterium after sterilization. However, these methods still have significant limitations: reliance on exogenous substrates or specific bacterial species increases costs and operational complexity; multi-stage processes extend processing time and increase facility investment; and processes based on sterilization and pure culture inoculation face challenges in contamination control and poor operational stability during large-scale applications. Overall, while pursuing propionic acid selectivity, these methods often introduce high process complexity and operating costs.

[0049] In the aforementioned technical pathways for the targeted preparation of propionic acid, pH control is widely used, but its adjustment range is usually cautiously limited to a range tolerable by the microorganisms, based on considerations of maintaining the activity of acid-producing bacteria (including propionic acid-generating bacteria). Current technical consensus holds that excessively high pH environments (e.g., pH greater than 12) will cause fatal stress to most microorganisms, including acid-producing bacteria, leading to the collapse of the fermentation system. Therefore, existing technical solutions actively avoid strongly alkaline regions when designing pretreatment or fermentation pH.

[0050] This invention uses a combination of extremely strong alkali pretreatment and pH adjustment to a suitable fermentation pH to deeply alter the physicochemical properties of the substrate and the microbial ecosystem, thereby obtaining a mixture of volatile fatty acids with propionic acid as the main target product through directional fermentation in kitchen waste.

[0051] When this method is applied, by implementing the strong alkali pretreatment and pH adjustment steps and controlling the pH of the strong alkali pretreatment at 13.3-13.5, a fermentation environment conducive to propionic acid-producing bacteria becoming the dominant microbial population can be specifically created, thereby significantly increasing the proportion of propionic acid in the total volatile fatty acids in the final fermentation product, for example, reaching more than 50%, thus achieving efficient and targeted conversion from kitchen waste to high-purity propionic acid precursor liquid.

[0052] The following describes embodiments of the present invention. These embodiments are exemplary and are only used to explain the present invention, and should not be construed as limiting the invention. Where techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0053] Example 1 refer to Figure 1 This embodiment describes a method for highly selectively producing propionic acid from kitchen waste through alkali pretreatment, which involves the following steps: Step 1: Adjust the pH value of food waste (TS = 8.26%, VS = 5.83%) to 13.3, denoted as the alkaline pretreatment pH. Then stir for 0.5 hours at this pH respectively to obtain pretreated food waste; Step 2: Callback the pH of the above pretreated food waste to 10, denoted as the post-callback fermentation pH; Step 3: Mix the pretreated food waste adjusted to the post-callback fermentation pH and the inoculum and add them into the reactor. The inoculum is the inoculated sludge (anaerobic granular activated sludge taken from the PTA wastewater treatment plant of Zhangzhou FuhaiChuang Petrochemical Co., Ltd.) pretreated by preheating at 90 - 105 °C for 1 - 2 hours. The TS of the inoculum after preheating treatment is: 10% < TS < 30%. The volume-mass ratio of the pretreated food waste adjusted to the post-callback fermentation pH to the inoculum is 12:1. Conduct nitrogen purging for 5 min, seal the reactor, and place it in an incubator at 35 °C for acid fermentation; Step 4: Ferment for 10 days, and sample every 2 days to detect the acid production results.

[0054] Reference Figures 2 to 4 , and the results show that the highest propionic acid selectivity reaches 52.1% after 4 days of fermentation, the propionic acid production reaches 12.1 gCOD / L, and the production rate is 184.5 mg COD / g VS.

[0055] Example 2 The difference between this example and Example 1 is that the alkaline pretreatment pH is adjusted to 13.5. Reference Figures 2 to 4 , and the results show that the highest propionic acid selectivity reaches 49.3% after 4 days of fermentation, the propionic acid production reaches 9.2 g COD / L, and the production rate is 140.2 mg COD / gVS.

[0056] Example 3 The difference between this example and Example 1 is that the alkaline pretreatment pH is adjusted to 13.3, and the post-callback fermentation pH is adjusted to 8. Reference Figures 2 to 4 , and the results show that the highest propionic acid selectivity reaches 50.3% after 4 days of fermentation, the propionic acid production reaches 10.6 g COD / L, and the production rate is 161.7 mg COD / g VS.

[0057] Comparative Example 1 The difference between this comparative example and Example 1 is that the alkaline pretreatment pH is adjusted to 10, and the post-callback fermentation pH adjustment is not carried out. Reference Figures 2 to 4 , and the results show that the highest propionic acid selectivity reaches 24.5% after 4 days of fermentation, the propionic acid production reaches 7.0 g COD / L, and the production rate is 107.1 mg COD / g VS.

[0058] Comparative Example 2 The difference between this comparative example and Example 1 is that the pH of the alkaline pretreatment was adjusted to 12. (Reference) Figures 2 to 4 The results showed that after 4 days of fermentation, the selectivity of propionic acid reached 25.9%, the yield of propionic acid reached 8.3 g COD / L, and the yield was 127.8 mg COD / g VS.

[0059] Comparative Example 3 The difference between this comparative example and Example 1 is that the pH of the alkaline pretreatment was adjusted to 13. (Reference) Figures 2 to 4 The results showed that after 4 days of fermentation, the selectivity of propionic acid reached 39.9%, the yield of propionic acid reached 10.2 g COD / L, and the yield was 155.8 mg COD / gVS.

[0060] Example 1 (pretreatment pH 13.3, pH adjustment 10) showed significantly better propionic acid selectivity (52.1%), yield (12.1 g COD / L), and production rate (184.5 mg COD / g VS) than Examples 2 (pretreatment pH 13.5, pH adjustment 10) and 3 (pretreatment pH 13.3, pH adjustment 8). Compared to Example 2, the excessively high pretreatment pH (13.5), while maintaining high selectivity, may lead to excessive hydrolysis or inhibition of some substrates, resulting in a significant decrease in propionic acid yield and production rate. Compared to Example 3, under the same pretreatment conditions, adjusting the pH to 8 still yielded high propionic acid selectivity, but the yield and production rate were lower than those adjusted to pH 10. This demonstrates that adjusting the pH to 10 better balances microbial activity and product accumulation in this system, and is one of the keys to achieving optimal propionic acid production performance.

[0061] Example 1 (pretreatment pH 13.3, pullback pH 10) showed significantly better propionic acid production performance than Comparative Example 1 (pretreatment pH 10, no pullback), Comparative Example 2 (pretreatment pH 12, pullback pH 10), and Comparative Example 3 (pretreatment pH 13, pullback pH 10). The comparison with Comparative Examples 1 and 2 directly demonstrates that when the pretreatment pH is below 13.3 (especially ≤12), high selective enrichment of propionic acid cannot be achieved, and the product remains predominantly a mixed acid. The comparison with Comparative Example 3 further shows that while the propionic acid selectivity (39.9%) and yield were higher than under lower pH conditions at a pretreatment pH of 13, they were still significantly lower than in Example 1 (pH 13.3). This confirms that controlling the pH of the strong alkaline pretreatment to 13.3-13.5 can directionally create a fermentation environment conducive to propionic acid-producing bacteria becoming the dominant flora, thereby significantly increasing the proportion of propionic acid in the total volatile fatty acids of the final fermentation product.

[0062] Furthermore, those skilled in the art should understand that although many problems exist in the prior art, each embodiment or technical solution of the present invention can be improved in only one or a few aspects, without necessarily solving all the technical problems listed in the prior art or the background art simultaneously. Those skilled in the art should understand that any content not mentioned in a claim should not be construed as a limitation on that claim.

[0063] Although this document frequently uses terms such as alkaline pretreatment pH, post-fermentation pH, inoculum, and controlled environmental conditions, the possibility of using other terms is not excluded. These terms are used merely for the convenience of describing and explaining the essence of the invention; interpreting them as any additional limitation would contradict the spirit of the invention. The terms "first," "second," etc. (if present) in the specification, claims, and accompanying drawings of the embodiments of the invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing volatile fatty acids through fermentation of kitchen waste, characterized in that, Includes the following steps: Alkalinization pretreatment is carried out on the food waste material to raise its pH value to the alkaline pretreatment pH, and the first stage of alkaline hydrolysis reaction is carried out under this condition to obtain the pretreated food waste substrate; The pH value of the pretreated kitchen waste substrate was adjusted to the pH value after the initial fermentation, and then mixed with the pretreated inoculum to carry out the second stage of anaerobic fermentation under controlled environmental conditions. The pH of the alkaline pretreatment is higher than the pH of the post-fermentation after the adjustment.

2. The method for preparing volatile fatty acids by fermentation of kitchen waste according to claim 1, characterized in that, The alkaline pretreatment pH is above 13.3; Preferably, the pH of the alkaline pretreatment is 13.3 to 13.

5.

3. The method for preparing volatile fatty acids by fermentation of kitchen waste according to claim 1, characterized in that, The post-fermentation pH after the pullback is 8.0 to 10.

0.

4. The method for preparing volatile fatty acids by fermentation of kitchen waste according to claim 1, characterized in that, The duration of the first stage alkaline hydrolysis reaction is 0.5 hours to 1 hour.

5. The method for preparing volatile fatty acids by fermentation of kitchen waste according to claim 1, characterized in that, The inoculum is preheated anaerobic sludge. Preferably, the temperature range of the preheating treatment is 90°C to 105°C, and the treatment time is 1 hour to 2 hours.

6. The method for preparing volatile fatty acids by fermentation of kitchen waste according to claim 5, characterized in that, The total solids content of the inoculum after preheating is 10% to 30%.

7. The method for preparing volatile fatty acids by fermentation of kitchen waste according to claim 1, characterized in that, The ratio of the pretreated food waste substrate to the inoculum is 1g of inoculum for every 10 to 12ml of substrate.

8. The method for preparing volatile fatty acids by fermentation of kitchen waste according to claim 1, characterized in that, The second stage of anaerobic fermentation is carried out under an inert gas atmosphere.

9. The method for preparing volatile fatty acids by fermentation of kitchen waste according to claim 1, characterized in that, The second stage of anaerobic fermentation is carried out in a closed reactor; Preferably, the reaction temperature of the second-stage anaerobic fermentation reaction is 35°C to 36°C; Preferably, the total duration of the second-stage anaerobic fermentation reaction is 4 to 10 days.

10. The application of the method according to any one of claims 1-9 in the directional fermentation of kitchen waste to prepare propionic acid.

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